The overall aim of this proposal is the understanding of the electronic structures of the constituents of reaction centers in the photosynthetic apparatus of bacterial systems and chloroplasts with emphasis on the former. The electronic wave-functions of the primary donor and intermediate and primary acceptors and the related systems, monomer cations and anions and triplet states of bacteriochlorophyll (BChl), chlorophyll (Chl) and derivatives will be obtained by molecular orbital techniques including pi and sigma orbitals. We shall concentrate on the interpretations of the variety of magnetic properties and nuclear hyperfine interactions obtainable by magnetic resonance and Mossbauer techniques. The objective of the hyperfine studies in the monomer cations and anions and dimer cations, which will include the influence of various important mechanisms such as exchange polarization and electron-electron interactions, will be to understand the available hyperfine data quantitatively, their trends among monomers and the observed ratios between "special pair" dimer and monomer cation data using current models for dimers. For the triplet states of both monomers and aggregates in vivo, we shall attempt to understand quantitatively the data on zero-field splitting and singlet-triplet crossing parameters and the relationship between the results for monomers and the corresponding aggregation PR, photosystems I and II and antenna chlorophylls. For primary acceptors, we shall attempt quantitative interpretations of observed resonance line-widths and g-shifts for the reduced quinone component and its interaction with the Fe 2 ion component, and for the latter the zero-field splitting parameters and Fe57m quadrupole coupling and isomer shift data from Mossbauer measurements. The influence of the interaction, of electrostatic, magnetic or covalent bonding types, between the two components on their respective properties shall be studied. For the intermediate acceptor I, assuming it to be either a BChl or a bacteriopheophytin (BPh) monomer, we shall study the interaction of I- with the reduced quinone and Fe 2 ion components of the primary acceptor to understand the line shape of the I- responance signal. In addition, our investigations on the hyperfine interactions in BChla- and BPha-monomers are expected to help in the identification of I-.